Gas hydrates have been known for many years. These hydrates are inclusion compounds wherein various light hydrocarbon gases or other gases, such as natural gas, methane, ethane, propane, butane, carbon dioxide, hydrogen sulfide, nitrogen, and combinations thereof, physically react with water at elevated pressures and low temperatures. The gas becomes included or entrapped within the extended solid water lattice network which includes hydrogen bonded water molecules. The hydrate structure is stable due to weak van der Waals' forces between the gas and water molecules and hydrogen bonding between water molecules within the lattice structure.
At least two different hydrate crystal structures are known, each of which is a clathrate crystalline structure. A clathrate hydrate unit crystal of structure I includes two tetrakaidecahedron cavities and six dodecahedron cavities for every 46 water molecules. A clathrate hydrate unit crystal of structure II contains eight large hexakaidecahedron cavities and 16 dodecahedron cavities for every 136 water molecules. A relatively large volume of gas can be entrapped under pressure in these cavities. For example, it has been determined that natural gas hydrates can contain as much as 180 standard cubic feet of gas per cubic foot of the solid natural gas hydrates.
Early on, gas hydrates were considered an industrial nuisance. Petroleum and natural gas production facilities are often located in cold environments, where the product is located in deep underground or underwater wells. When tapping these wells, all of the necessary ingredients and conditions are present for producing gas hydrates--i.e., light hydrocarbon gases and water are present, the temperature is low, and the pressure is high. Therefore, gas hydrates were often produced spontaneously in the drilling and transmission pipes during oil or natural gas production. Because gas hydrates are solid materials that do not readily flow in concentrated slurries or in solid form, when spontaneously produced during oil or natural gas production, the hydrates tend to clog the pipes, channels, and equipment in the production and transmission systems. These disadvantageous properties of gas hydrates spawned much research into methods for inhibiting hydrate formation and eliminating this nuisance. See, for example, D. Katz, et al., Handbook of Natural Gas, McGraw-Hill, New York (1959) pp. 189-221; E. D. Sloan, Jr., Clathrate Hydrates of Natural Gases, Marcel Dekker, Inc. (1991). These documents are entirely incorporated herein by reference.
But, because of the relatively high volume of gas that potentially can be stored in gas hydrates, eventually researchers began to look at this "nuisance" as a possible method for storing and/or transporting gases. See B. Miller, et al., Am. Gas. Assoc. Mon. Vol. 28, No. 2 (1946), pg. 63. This document is entirely incorporated herein by reference. U.S. Pat. No. 3,514,274 to Cahn, et al., also entirely incorporated herein by reference, describes a process in which a solid hydrate phase is generated in one or more process steps. The hydrate then is conveyed to storage or a marine transport vessel. This process is disadvantageous, however, because it requires conveyance of a concentrated hydrate slurry in a liquid propane carrier.
Hutchinson, et al., U.S. Pat. No. 2,375,559 (which patent is entirely incorporated herein by reference), describe a process for hydrating hydrocarbon gases. In this process, the gas and water components are mixed in a pipe that moves the hydrate product to storage tanks. Because of the poor flowing properties of gas hydrates, as noted above, this device would be subject to clogging.
U.S. Pat. No. 2,904,511 to Donath illustrates a water desalination apparatus that produces desalinated water from salt water by forming gas hydrates. The hydrate forming vessel of Donath is partially filled with water to be purified, and the hydrate-forming gas is introduced into the liquid water to form the hydrate. Because of the presence of the liquid water in the hydrate forming vessel, this apparatus would not be well suited for use on board a ship or oil platform or other areas influenced by waves. This Donath patent is entirely incorporated herein by reference.
Gudmundsson describes various systems for making gas hydrates. See, for example, U.S. Pat. No. 5,536,893; WO Patent Publication No. 93/01153; "Transport of Natural Gas as Frozen Hydrate," ISOPE Conference Proceedings, V1, The Hague, Netherlands, June 1995; and "Storing Natural Gas as Frozen Hydrate," SPE Production & Facilities, February 1994. These documents each are entirely incorporated herein by reference. A typical system 100 of Gudmundsson is generally shown in FIG. 1. In this system, natural gas from a gas source G is compressed (102), cooled (104), and fed to a continuously stirred tank reactor vessel 106. Water from a suitable source S is pumped (pump "P") through a cooler 108 to form a water/ice slurry that is introduced into the tank 106. The tank 106 is maintained under conditions appropriate to produce a gas hydrate (e.g., 50.degree. F., 720 psig). The gas hydrate slurry produced in the tank 106 is transported via line 110 to a separator 112 where water is removed via line 114. The separator 112 includes a series of cyclones and a rotary-drum filter. Finally, the purified hydrates are frozen to 5.degree. F. in a freezer 116, from where the hydrates 118 are transferred to storage or a transport device.